html/template: reenable testcases and fix mis-escaped sequences.
html/template: reenable testcases and fix mis-escaped sequences.

Tighter octal parsing broke some tests and were disabled in
http://codereview.appspot.com/5530051

Those tests were broken.  The CSS decoder was supposed to see CSS
hex escape sequences of the form '\' <hex>+, but those escape
sequences were instead being consumed by the template parser.

This change properly escapes those escape sequences, and uses
proper escaping for NULs.

R=golang-dev, rsc, nigeltao
CC=golang-dev
http://codereview.appspot.com/5529073

Triskite/willstone-goclone,webfd/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,Triskite/willstone-goclone,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,Triskite/willstone-goclone,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rdp/rogerpack2005-golang,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,scirelli/scirelli-go